1. Field of the Invention
The present invention relates generally to a method and apparatus for transmitting control information in a mobile communication system, and in particular, to a method and apparatus for transmitting uplink acknowledge (ACK) information in an Orthogonal Frequency Division Multiple Access (OFDMA) communication system.
2. Description of the Related Art
Mobile communication systems are evolving into a 4th generation (4G) mobile communication system supporting a super high-speed multimedia service, following a 1st generation (1G) analog system, a 2nd generation (2G) digital system, and a 3rd generation (3G) IMT-2000 system supporting a high-speed multimedia service. In the 4G mobile communication system, a user can access a satellite network, a local area network (LAN), and an Internet network with one terminal. That is, the user is provided with many kinds of services, such as voice, image, multimedia, Internet data, voice mail, and instant message services, with one mobile terminal. More specifically, the 4G mobile communication system provides a data rate of 20 Mbps for a super high-speed multimedia service, and commonly uses an Orthogonal Frequency Division Multiplexing (OFDM) scheme.
The OFDM scheme, which is a digital modulation scheme for multiplexing multiple orthogonal carrier signals, divides a single data stream into several low-speed streams and simultaneously transmits the low-speed streams using several subcarriers with a low data rate.
A multiple access scheme based on the OFDM scheme is known as an Orthogonal Frequency Division Multiple Access (OFDMA) scheme. In the OFDMA scheme, subcarriers in one OFDM symbol are shared by a plurality of users, i.e., subscriber terminals. A communication system based on the OFDMA scheme (hereinafter referred to as an “OFDMA communication system”) has separate physical channels for transmitting control information. Uplink control information, which is a type of the control information, includes Channel Quality Indicator (CQI), acknowledge/non-acknowledge (ACK/NAK), Coefficient for Multi-Input Multi-Output (MIMO) system, etc.
FIG. 1 is a diagram illustrating a transmitter for transmitting uplink ACK information in an OFDMA communication system according to the prior art. The uplink ACK information shall be ACK if the corresponding downlink packet has been successfully received; otherwise, it shall be NAK. Referring to FIG. 1, a transmitter 10 includes a binary channel encoder 11, a modulator 12, and an inverse fast Fourier transform (IFFT) block 13.
If there are information data bits for an uplink ACK to be transmitted, the binary channel encoder 11 encodes the information data bits into a codeword using a binary block code, for example, a (20,5) block code.
The modulator 12 includes a coherent modulator or a differential modulator. The modulator 12 determines a transmission symbol corresponding to the codeword output from the binary channel encoder 11 using a coherent or differential modulation scheme, and outputs the transmission symbol to the IFFT block 13. The modulator 12 can use, for example, a Quadrature Phase Shift Keying (QPSK) scheme or a Differential Quadrature Phase Shift Keying (DQPSK) scheme.
The IFFT block 13 performs IFFT on the transmission symbol output from the modulator 12, and transmits the IFFF-processed transmission symbol.
FIG. 2 is a diagram illustrating a receiver for receiving uplink ACK information in an OFDMA communication system according to the prior art. Referring to FIG. 2, a receiver 20 includes a fast Fourier transform (FFT) block 23, a demodulator 22, and a binary channel decoder 21.
Upon receiving a signal transmitted from the transmitter 10, the FFT block 23 performs FFT on the received signal and outputs a received symbol to the demodulator 22. The demodulator 22 includes a coherent demodulator or a differential demodulator. The demodulator 22 receives the received symbol output from the IFFT block 23, and calculates a soft decision value thereof using a demodulation scheme corresponding to the modulation scheme used in the transmitter 10, e.g., coherent demodulation or differential demodulation.
The binary channel decoder 21 receives the soft decision value calculated by the demodulator 22, determines which codeword was transmitted, and outputs data bits corresponding thereto.
The uplink ACK information exchanged between the transmitter 10 and the receiver 20 is not large in the amount for the overall communication services. For example, the ACK information is only one bit. However, because the uplink ACK information is very important information for operation of the communication system, highly reliable transmission should be guaranteed for the uplink ACK information. However, it is common that few frequency-time resources are allocated to physical channels used for transmitting the uplink ACK in order to reduce an overhead rate. Therefore, there is a need for a new transmission method, which is different from the channels to which many resources are allocated, and that transmits a large volume of information, like the traffic channel.
Generally, a combined method of a binary channel code and coherent modulation or differential modulation is used to transmit an uplink ACK channel. The uplink ACK channel is a unit consisting of a number of subcarriers for transmitting the uplink information.
However, when the uplink ACK channel is transmitted using fewer frequency-time resources in this method, an error rate increases, thereby decreasing operation stability of the communication system. That is, whereas sufficient pilot tones for downlink or transmission of uplink traffic area available, there are insufficient traffic tones for transmission of the uplink ACK information. The lack of pilot tones deteriorates channel estimation performance, thereby degrading the performance of a coherent modulation/demodulation scheme. If the number of pilot tones is increased, considering only the channel estimation performance, the number of data tones becomes insufficient.
In addition, separation of the binary channel code and the modulation reduces the optimized performance.
Further, if many frequency-time resources are used for transmission of an uplink ACK information, in order to increase the stability, the overhead rate increases, which reduces throughput of the communication system.